In recent years, display devices have been progressed from display devices using cathode ray tubes to flat panel display devices referred to as FPD such as liquid crystal displays or plasma displays. In FPD, thin film transistors have been adopted as switching elements concerned with display switching by liquid crystals. For example, as the switching element for the liquid crystal displays, thin film transistors in which amorphous silicon or crystal silicon is applied to a channel layer have been adopted. For the FPD, provision of new functions such as large area, high-definition, flexible configuration, etc. have been demanded and the adaptability has been demanded also for a process that enables manufacture of FPD of a large area or flexible substrate as well as high performance as an image element device.
Further, organic EL displays utilizing organic EL has also been developed recently with an aim for attaining larger area or flexible structure. Also in the organic EL display devices, thin film transistors have been adopted as the switching element. However, since the organic EL display is a self-emitting device in which an organic semiconductor layer is driven to emit light, characteristics as a current driving device have been demanded for the thin film transistor unlike the case in the liquid crystal displays.
With the background as described above, for improving the transistor characteristics of thin film transistors intended for display devices, application of an oxide semiconductor having a band gap as large as about 3 eV, being transparent, and capable of being formed at a low temperature to a channel layer of a thin film transistor has been investigated. For the oxide semiconductor, it has been expected for the application use to thin film memories, RFIDs, etc. in addition to the display devices.
As the oxide semiconductor forming a channel layer of the thin film transistor, zinc oxide (ZnO) and tin oxide (SnOx) have been used generally but the thin film transistors using them involve a problem that a threshold voltage tends to fluctuate. Then, as an oxide semiconductor capable of suppressing the fluctuation of the threshold voltage of the thin film transistor, an indium gallium zinc complex oxide (IGZO) has been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2006-165532 (Patent Literature 1) discloses a semiconductor device having a P-type region and an N-type region and using an amorphous oxide at an electron carrier concentration of less than 1018/cm3 for the N-type region in which the amorphous oxide is an oxide including indium (In), gallium (Ga), and zinc (Zn).
Further, Japanese Unexamined Patent Application Publication No. 2006-173580 (Patent Literature 2) describes a field effect transistor having an active layer including an amorphous oxide at an electron carrier concentration of less than 1018/cm3, or an amorphous oxide that tends to increase the electron mobility along with increase in the electron carrier concentration, in which at least one of a source electrode, a drain electrode, and a gate electrode has permeability to light in a visible region and the amorphous oxide is an oxide containing In, Zn, and Ga.
In a thin film transistor in which IGZO is applied to a channel layer, a more preferred value of sub-threshold slope than that of a thin film transistor in which poly silicon is applied to a channel layer has been confirmed. Further, IGZO is expected to have application use not only to display devices but also to other devices requiring ultra low voltage operation or ultra low power consumption. On the other hand, however, since IGZO contains a considerable amount of In or Ga involving the possibility of depletion of resources or maldistribution of resources, it is considered that IGZO is not advantageous in the future industrial use.
Then, the inventors of the present application have investigated a zinc tin complex oxide (ZTO) as an oxide semiconductor not utilizing In. As a result, in a thin film transistor in which IGZO is applied to the channel layer, transistor characteristics comparable with those of thin film transistor in which IGZO is applied to a channel layer could be obtained. Further, it is estimated that there may be a great demand also for oxide semiconductor targets used upon deposition of oxide semiconductors by a sputtering method or a physical vapor deposition method in the feature and it is considered that a material such as ZTO not using a rare earth metal is superior to the material using the rare earth metal such as In or Ga also in view of the cost or sustainability of resources.